1. Field of the Invention
The present invention relates to the field of plant improvement via genetic engineering. More particularly, the present invention relates to the modification of plant growth and structure by molecular biological means to enhance the yield, in particular the yield of harvestable components, of agronomic and horticultural crop plants grown in the field. This is achieved by providing transgenic plants in which shade avoidance reactions, which normally occur in dense crop stands, and which are mediated by the phytochrome system, are controlled. In this way, wasteful redirection of assimilates to stem growth, production of elongated plants susceptible to lodging, and reduction of harvest index can, in turn, be controlled, thereby providing plants with improved performance characteristics.
2. Description of Related Art
Phytochrome A (which is encoded by PHYA; see below) accumulates in etiolated wild-type (WT) seedlings to high levels, where it mediates stem growth inhibition in response to light, the wavelength maximum being in the far-red (FR=700-800 nm region of the photospectrum). Phytochrome B (which is encoded by PHYB; see below) acts principally in light-grown plants, where it modulates stem growth as a function of the relative amounts of red (R=600-700 nm) and FR radiation. When the proportion of FR is high, as in plant canopies, phytochrome B induces the shade avoidance syndrome, involving marked increases in stem extension and concomitant decreases in the growth of other organs, e.g. harvestable components such as leaf, fruit and storage organs. Normally, phytochrome A is not active in light-grown plants as it is rapidly degraded and its synthesis down-regulated in plants exposed to light (Quail, P. H., 1994, In Photomorphogenesis in Plants, R. E. Kendrick and G. H. M. Kronenberg, Eds., Second Edition, Kluwer Academic Publishers, Dordrecht, pps. 71-104). Transgenic plants expressing introduced PHYA cDNAs at high levels are severely dwarfed (McCormac, A. C. et al., 1992, Planta, 188: 173-181; Cherry, J. R. et al., 1991, Plant Physiol., 96: 775-785).
Shade avoidance is a mechanism whereby, in an attempt to out-compete their neighbors, plants grown in close proximity respond to far-red (FR) radiation reflected from the leaves of neighboring plants by increasing significantly their stem length at the expense of leaf, fruit and storage organ development, thereby adversely affecting the yield of harvestable components (Smith, H., 1982, Ann. Rev. Pl. Physiol., 33: 481-518; Schmitt, J. M. and Wulff, R. D., Trends in Ecology and Evolution, 8: 47-51; Ballare, C. L. et al., 1990, Science, 247: 329-332; Smith, H., 1995, Ann. Dev. Pl. Physiol. Mol. Biol., 46: 289-315; Schmitt, J. M. et al., 1995, American Naturalist, 146: 937-953).
This proximity perception and response is mediated primarily by phytochrome B (Smith, H. and Whitelam, G. C., 1990, Plant Cell Environ., 13: 696-707; Whitelam, G. C. and Harberd, N. P., 1994, Plant Cell Environ., 17: 615-625), which is encoded by the PHYB gene, a member of the small, diverse PHY multigene family (Sharrock, R. A. and Quail, P. H., 1989, Genes Devel., 3: 534-544); phytochrome D may also be involved in shade avoidance responses (Robson, P. H. R. and Smith, H. unpublished data). Phytochrome A (which is encoded by the PHYA gene) is not normally involved in shade avoidance (Whitelam, G. C. et al., 1993, Plant Cell, 5: 757-768), but when overexpressed in transgenic plants can antagonise the action of phytochrome B (McCormac, A. C. et al., 1992, supra; Smith, H., 1994, Seminars in Cell Biology, 5: 315-325). Experiments performed to date (McCormac, A. C. et al., 1992, supra), have only shown that transgenic plants expressing introduced PHYA cDNA at high levels are severely dwarfed, irrespective of the proximity of neighboring plants.